Domestic cats are subject to infection by several retroviruses, including feline leukemia virus (FeLV), feline sarcoma virus (FeSV), endogenous type C oncoronavirus (RD-114), and feline syncytia-forming virus (FeSFV). Of these, FeLV is the most significant pathogen, causing diverse symptoms including lymphoreticular and myeloid neoplasms, anemias, immune-mediated disorders, and an immunodeficiency syndrome that is similar to human acquired immune deficiency syndrome (AIDS). Recently, a particular replication-defective FeLV mutant, designated FeLV-AIDS, has been more particularly associated with immunosuppressive properties.
The discovery of feline T-lymphotropic lentivirus (now designated as feline immunodeficiency virus, FIV) was first reported in Pedersen et al. (1987). Characteristics of FIV have been reported in Yamamoto et al. (1988a); Yamamoto et al. (1988b); and Ackley et al. (1990). Seroepidemiologic data have shown that infection by FIV is indigenous to domestic and wild felines throughout the world. A wide variety of symptoms are associated with infection by FIV, including abortion, alopecia, anemia, conjunctivitis, chronic rhinitis, enteritis, gingivitis, hematochezia, neurologic abnormalities, periodontitis, and seborrheic dermatitis. The immunologic hallmark of domestic cats infected with FIV is a chronic and progressive depletion of feline CD4.sup.+ peripheral blood lymphocytes, a reduction in the CD4:CD8 cell ratio and, in some cases, an increase in CD8-bearing lymphocytes. Based on molecular, biochemical and immunopathologic characteristics, FIV infection of cats is now considered to be a better feline AIDS model than FeLV-FAIDS.
Cloning and sequence analysis of FIV has been reported in Olmsted et al. (1989a); Olmsted et al. (1989b); and Talbott et al. (1989). Hosie and Jarret (1990) described the serological response of cats infected with FIV. FIV virus subtypes can be classified according to immunotype based on the level of cross-neutralizing antibodies elicited by each strain (Murphy and Kingsbury, 1990). Recently, viruses have been classified into subtypes according to genotype based on nucleotide sequence homology. Although HIV and FIV subtyping is based on genotype (Sodora et al., 1994; Rigby et al., 1993; and Louwagie et al., 1993), little is known about the correlation between the genotype and immunotype of subtypes. FIV viral isolates are currently classified into four FIV subtypes: A, B, C and D. (Kakinuma et al., 1995). Infectious isolates and infectious molecular clones have been described for all FIV subtypes except for subtype C (Sodora et al., 1994). Subtype C FIV has only been identified from cellular DNA of cats from Canada (Sodora et al., 1994; Rigby et al., 1993; Kakinuma et al., 1995).
A major difficulty in developing an FIV vaccine has been in identifying a vaccine approach that is effective against a broad range of FIV strains including field isolates from different subtypes or clades. Vaccine prophylaxis for FIV has been attained against homologous and slightly heterologous strains using a single-strain vaccine, but not against challenge with moderate to greatly heterologous strains (Johnson et al., 1994; Yamamoto et al., 1993). Thus, there remains a need for a vaccine that protects across multiple FIV subtypes.